Carbon seal assembly

ABSTRACT

A seal assembly includes a housing that includes a seal flange at least partially defining a seal opening. A carbon seal is located at least partially in the seal opening and includes a first axially facing surface. The seal flange includes an axially facing surface that has a carbide based coating and a diamond-like carbon coating in engagement with the first axially facing surface on the carbon seal.

BACKGROUND

The present disclosure relates to seals and, more particularly, tocarbon seals used in gas turbine engines.

A gas turbine engine typically includes a fan section, a compressorsection, a combustor section, and a turbine section. Air entering thecompressor section is compressed and delivered into the combustorsection where it is mixed with fuel and ignited to generate a high-speedexhaust gas flow. The high-speed exhaust gas flow expands through theturbine section to drive the compressor and the fan section.

In one example, circumferential carbon seals are used in bearingcompartments of gas turbine engines to provide a seal between oil usedto lubricate parts of the gas turbine engine and other parts of the gasturbine engine. Bearing assemblies may also support a rotating shaft ofthe gas turbine engine. The circumferential carbon seals may be sealedagainst a seal carrier or housing against rotating seal components ofthe gas turbine engine.

SUMMARY

In one exemplary embodiment, a seal assembly includes a housing thatincludes a seal flange at least partially defining a seal opening. Acarbon seal is located at least partially in the seal opening andincludes a first axially facing surface. The seal flange includes anaxially facing surface that has a carbide based coating and adiamond-like carbon coating in engagement with the first axially facingsurface on the carbon seal.

In a further embodiment of any of the above, the carbide based coatingis in direct contact with the seal flange.

In a further embodiment of any of the above, the diamond-like carboncoating is in direct contact with the carbide based coating on thecarbon seal.

In a further embodiment of any of the above, the carbide based coatingincludes at least one of a tungsten carbide-cobalt or a chromiumcarbide.

In a further embodiment of any of the above, the carbide based coatingand the diamond-like carbon coating extend circumferentially with theflange.

In a further embodiment of any of the above, the carbide based coatingand the diamond-like carbon coating create an axial separation betweenthe flange and the carbon seal.

In a further embodiment of any of the above, the carbon seal includes anelectrocarbon grade carbon.

In a further embodiment of any of the above, the diamond-like carboncoating is silicon doped.

In a further embodiment of any of the above, the carbide based and thediamond-like carbon coating separate the housing from contacting thecarbon seal.

In a further embodiment of any of the above, a shaft is located adjacenta radially inner contact surface of the carbon seal.

In a further embodiment of any of the above, the carbon seal includes afirst axial dimension and the radially inner contact surface includes asecond axial dimension less than the first axial dimension.

In a further embodiment of any of the above, the radially inner contactsurface is located on an inner side of a projection of the carbon seal.

In a further embodiment of any of the above, the carbide based coatingis between 76 and 152 micrometers thick.

In a further embodiment of any of the above, the diamond-like carboncoating is approximately 1 micrometer thick.

In another exemplary embodiment, a method of using a seal assemblyincludes locating a carbon seal adjacent a housing and separating thecarbon seal from the housing with a carbide based coating and adiamond-like carbon coating. A lubricant is generated between the carbonseal and the diamond-like carbon coating.

In a further embodiment of any of the above, the carbide based coatingis in direct contact with the housing. The diamond-like carbon coatingis at least partially spaced from the housing by the carbide. Thecarbide based coating includes at least one of tungsten carbide-cobaltor chromium carbide.

In a further embodiment of any of the above, generating the lubricantincludes forming a graphitic material between the interface of thecarbon seal and the diamond-like carbon coating.

In a further embodiment of any of the above, a shaft is rotated adjacentthe carbon seal and it includes a velocity of at least 152 m/s relativeto the carbon seal.

In a further embodiment of any of the above, the carbide based coatingis applied through at least one of a high velocity oxygen fuel coatingprocess or an APS coating process.

In a further embodiment of any of the above, the diamond-like carbon isapplied through at least one of a PVD or CVD process.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 is a schematic view of a non-limiting example of a gas turbineengine.

FIG. 2 illustrates an axial view of a shaft and a carbon seal assembly.

FIG. 3 illustrates a cross-sectional view taken along line 3-3 of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. The fan section 22 drivesair along a bypass flow path B in a bypass duct defined within a housing15, such as a fan case or nacelle, and also drives air along a core flowpath C for compression and communication into the combustor section 26then expansion through the turbine section 28. Although depicted as atwo-spool turbofan gas turbine engine in the disclosed non-limitingembodiment, it should be understood that the concepts described hereinare not limited to use with two-spool turbofans as the teachings may beapplied to other types of turbine engines including three-spoolarchitectures.

The exemplary engine 20 generally includes a low speed spool 30 and ahigh speed spool 32 mounted for rotation about an engine centrallongitudinal axis A relative to an engine static structure 36 viaseveral bearing systems 38. It should be understood that various bearingsystems 38 at various locations may alternatively or additionally beprovided, and the location of bearing systems 38 may be varied asappropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects, a first (or low) pressure compressor 44 and a first (orlow) pressure turbine 46. The inner shaft 40 is connected to the fan 42through a speed change mechanism, which in exemplary gas turbine engine20 is illustrated as a geared architecture 48 to drive a fan 42 at alower speed than the low speed spool 30. The high speed spool 32includes an outer shaft 50 that interconnects a second (or high)pressure compressor 52 and a second (or high) pressure turbine 54. Acombustor 56 is arranged in exemplary gas turbine 20 between the highpressure compressor 52 and the high pressure turbine 54. A mid-turbineframe 57 of the engine static structure 36 may be arranged generallybetween the high pressure turbine 54 and the low pressure turbine 46.The mid-turbine frame 57 further supports bearing systems 38 in theturbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and fan drive gear system 48 may be varied. For example,gear system 48 may be located aft of the low pressure compressor, or aftof the combustor section 26 or even aft of turbine section 28, and fan42 may be positioned forward or aft of the location of gear system 48.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than about ten (10), thegeared architecture 48 is an epicyclic gear train, such as a planetarygear system or other gear system, with a gear reduction ratio of greaterthan about 2.3 and the low pressure turbine 46 has a pressure ratio thatis greater than about five. In one disclosed embodiment, the engine 20bypass ratio is greater than about ten (10:1), the fan diameter issignificantly larger than that of the low pressure compressor 44, andthe low pressure turbine 46 has a pressure ratio that is greater thanabout five 5:1. Low pressure turbine 46 pressure ratio is pressuremeasured prior to inlet of low pressure turbine 46 as related to thepressure at the outlet of the low pressure turbine 46 prior to anexhaust nozzle. The geared architecture 48 may be an epicycle geartrain, such as a planetary gear system or other gear system, with a gearreduction ratio of greater than about 2.3:1 and less than about 5:1. Itshould be understood, however, that the above parameters are onlyexemplary of one embodiment of a geared architecture engine and that thepresent disclosure is applicable to other gas turbine engines includingdirect drive turbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and35,000 ft (10,668 meters), with the engine at its best fuelconsumption—also known as “bucket cruise Thrust Specific FuelConsumption (′TSFC)”—is the industry standard parameter of lbm of fuelbeing burned divided by lbf of thrust the engine produces at thatminimum point. “Low fan pressure ratio” is the pressure ratio across thefan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The lowfan pressure ratio as disclosed herein according to one non-limitingembodiment is less than about 1.45. “Low corrected fan tip speed” is theactual fan tip speed in ft/sec divided by an industry standardtemperature correction of [(Tram ° R)/(518.7° R)]^(0.5). The “Lowcorrected fan tip speed” as disclosed herein according to onenon-limiting embodiment is less than about 1150 ft/second (350.5meters/second).

As shown in FIGS. 2 and 3 , a carbon seal assembly 60 completelysurrounds the outer shaft 50. However, the carbon seal assembly 60 couldbe located around other shafts in the gas turbine engine 20, such as theinner shaft 40 or shafts within the geared architecture 48. The carbonseal assembly 60 at least includes a carrier or housing 62 that at leastpartially enclosing a carbon seal 64.

In the illustrated example, the housing 62 and the carbon seal 64 eachform a single continuous ring. However, in another example, the housing62 and the carbon seal 64 include multiple circumferential segmentsjointed together to form a ring with joints or discontinuities. Thecarbon seal 64 can be comprised of a suitable electrocarbon, such as anelectrographitic grade carbon, salt impregnated carbon-graphite, resinimpregnated carbon-graphite, non-impregnated carbon graphite, saltimpregnated electrographite, non-impregnated electrographite eitherentirely or partially.

As shown in FIG. 3 , the housing 62 includes an axially extendingportion 66 and a radially extending portion 68 that each form a ring. Inthe illustrated example, the radially extending portion 68 extendsradially inward from an axially forward end of the axially extendingportion 66 such that a radially inner end of the radially extendingportion 68 is a free end and spaced from the outer shaft 50. In thisdisclosure, radial or radially and axial or axially is in relation tothe central longitudinal axis A and upstream or downstream is inrelation to a flow of air through the gas turbine engine 20.Additionally, the fan section 22 is located adjacent an axially forwardor upstream end of the gas turbine engine 20 and the turbine section 28is located adjacent an axially aft or downstream end of the gas turbineengine 20.

The radially extending portion 68 forms a seal flange and includes anaxially forward surface 70 at least partially defining an oil retainingportion 72 that is separated from an air portion 74 on an opposite sideof the carbon seal assembly 60. An axially aft surface 76 is located onan opposite side of the radially extending portion 68 from the axiallyforward surface 70. The axially aft surface 76 is at least partiallycovered with multiple layers of coatings. A first coating is a carbidebased coating 78, such as tungsten carbide-cobalt or chromium carbide,that is directly in contact with the axially surface 76. A second layeris a diamond-like carbon (“DLC”) coating 80 that covers the carbidebased coating 78 and is at least partially spaced form the axially aftsurface 76 by the carbide based coating 78. Therefore, the carbon seal64 is spaced from and not in contact with the housing 62 due at least inpart to the carbide based coating 78 and the diamond-like carbon coating80. Both the carbide based coating 78 and the diamond-like carboncoating 80 extend circumferentially with the radially extending portion68 to form a complete ring of separation between the radially extendingsurface 68 and the carbon seal 64.

In the illustrated example, the carbide based coating 78 is applieddirectly to the axially aft surface 76 on the radially extending portion68 through one of a high velocity oxygen fuel coating process or an APScoating process. The diamond-like carbon coating 80 is spaced from theaxially aft surface 76 and the housing 62 by the carbide based coating78 and is applied through one of a physical vapor deposition (“PVD”) ora chemical vapor deposition (“CVD”) process. In the illustrated example,the thicknesses of the carbide based coating 78 and the diamond-likecarbon coating 80 are for illustrative purposes only and are not toscale. For example, the carbide based coating 78 can have a thickness ofbetween 76 and 152 micrometers (3-6 thousandths of an inch) and thediamond-like carbon coating 80 is approximately 1 micrometer (0.04thousandths of an inch).

The diamond-like carbon coating 80 is a thin film formed from a DLCmaterial having sp2 and sp3 content. The sp2 content is indicative ofgraphitic content of the material, while the sp3 content is indicativeof the diamond-like content of the material. In one non-limitingconfiguration, the sp2 content of the film material is greater than thesp3 content. Another aspect for characterizing the DLC material isreferred to as micro-Raman. Micro-Raman provides a ‘G’ peak and a ‘D’peak, which refer to disorder and graphite respectively. Thediamond-like carbon coating 80 film for use in the present disclosurecan have a I(D)/I(G) peak ratio of <or =1.0 based on micro-Ramananalysis.

In another non-limiting configuration, the diamond-like carbon coating80 can be doped with carbide-forming metals to improve wear resistanceof the film. Such carbide-forming metals can include tungsten or siliconor combinations thereof, and these metals help to form carbides in thefilm which increase wear resistance. In some instances, thecarbide-forming metal can also or in addition be chromium or molybdenumor combinations thereof, which can also assist in the formation ofcarbides.

It should be appreciated that the diamond-like carbon coating 80 inaccordance with the present disclosure produces a low friction andwear-resistant carbon-based seal interface which, for example, canoperate effectively between 200 and 350° F., under elevated slidingvelocities. This, in turn, can reduce sub-surface heating (for exampledue to frictional heating) by reducing the friction co-efficient andimproving the break-in phase, which will consequently improve long termwear resistance of the carbon seal 64.

The diamond-like carbon coating 80 creates a carbon-carbon interfacewith low friction from the beginning of operation, and thereforeproduces a very short break-in phase. During initial operation, atransfer film or graphitic material 92 is formed over the diamond-likecarbon coating 80. The graphitic material 92 remains through steadystate operation of the carbon seal 64 to reduce wear on the carbon seal64. The carbon seal 64 can also include film controllers to reduce thegeneration of excess graphitic material 92.

The carbon seal 64 is biased towards the outer shaft 50 with a garterspring 65 when the carbon seal 64 is segmented. The garter spring 65 islocated in a groove 84 in a radially outer surface of the carbon seal 64and surrounds the carbon seal 64 to apply a compressive force to thecarbon seal 64. One feature of the garter spring 65 is a reduction inleakage between the carbon seal 64 and the outer shaft 50 due to thecompressive force applied. The outer shaft 50 can include a velocityrelative to the carbon seal 64 of at least 152 m/s (approximately 500ft/s) or at least 183 m/s (approximately 600 ft/s).

In the illustrated example, the carbon seal 64 includes a projection 86adjacent the outer shaft 50 having a shaft contacting surface 90. Theprojection 86 forms a clearance gap 88 between the carbon seal 64 andthe outer shaft 50. The clearance gap 88 results from the projection 86having a smaller axial dimension than the carbon seal 64. In theillustrated example, the projection 86 includes an axial dimension P1extending from an axially forward most edge to an axially aft most edgeof the shaft contacting surface 90. The carbon seal 64 includes an axialdimension C1 that is greater than the axial dimension P1. In theillustrated example, the axial dimension P1 is less than 25% of theaxial dimension C1. In another example, the axial dimension P1 is lessthan 50% of the axial dimension C1.

During operation of the seal assembly 60 in the gas turbine engine 20,the carbon seal 64 can move relative to the housing 62. Without thecoatings 78, 80, the carbon seal 64 is in direct contact with themetallic housing 62 which can cause wear on an axially upstream side 67of the carbon seal 64. One feature of the coatings 78 and 80 is that thecarbon seal 64 contacts the diamond-like carbon coating 80 to create acarbon on carbon interface. Additionally, movement between the carbonseal 64 and the housing 62 results in the formation of the graphiticmaterial 92 between the diamond-like carbon coating 80 and the carbonseal 64 as described above. The graphitic material 92 reduces frictionalforces between the diamond-like carbon coating 80 and the carbon seal 64to reduce wear on the axially upstream side 67 of the carbon seal 64.Although the illustrated example shows the graphitic material 92 locatedbetween a portion of the diamond-like carbon coating 80 and the carbonseal 64, the graphitic material 92 can form in a majority of the contactarea between diamond-like carbon coating 80 and carbon seal 64.

The foregoing description shall be interpreted as illustrative and notin any limiting sense. A worker of ordinary skill in the art wouldunderstand that certain modifications could come within the scope ofthis disclosure. For these reasons, the following claim should bestudied to determine the true scope and content of this disclosure.

What is claimed is:
 1. A seal assembly comprising: a housing including aseal flange at least partially defining a seal opening; and a carbonseal located at least partially in the seal opening and including afirst axially facing surface and the carbon seal is fixed from rotationrelative to the housing; wherein the seal flange includes an axiallyfacing surface having a carbide based coating and a diamond-like carboncoating in engagement with the first axially facing surface on thecarbon seal; wherein the carbon seal includes a first axial dimensionand a sealing surface in engagement with a rotatable shaft includes asecond axial dimension that is less than the first axial dimension,wherein the sealing surface is a radially innermost surface of thecarbon seal; wherein the carbide based coating is in direct contact withthe seal flange; and wherein the diamond-like carbon coating is indirect contact with the carbide based coating at a coating interface andthe carbon seal and the diamond-like carbon coating is spaced from theseal flange by the carbide based coating.